1) Field of the Invention
The present invention relates to an anti-lock brake system and, more particularly, to a solenoid valve for an anti-lock brake system that can enhance a braking performance by variably controlling an amount of fluid to be supplied during a slip control.
2) Description of Related Art
Generally, a brake system comprises a wheel cylinder mounted on an automotive wheel to generate brake force using hydraulic pressure, a booster/mater cylinder assembly for forming hydraulic pressure and supplying the same to the wheel cylinder, and a modulator for controlling the hydraulic pressure to be supplied according to a running condition of a vehicle in accordance with signals transmitted from an electronic control unit.
The modulator is equipped with a normal-open and normal-close solenoid valves for controlling the supply of brake pressure. The normal-open solenoid valve is designed to, in a normal brake mode, maintain an opened state to supply hydraulic pressure to the master cylinder without pressure reduction, and, in a slip control mode, control an amount of the oil to control the brake pressure.
The normal-open solenoid valve controls the amount of the oil by a simple On/Off operation of a plunger. Accordingly, the flow rate of the oil which can pass through the valve at the slip control mode is the same as at the normal brake mode.
Therefore, in such a conventional normal-open solenoid valve, the hydraulic pressure to be supplied cannot be precisely controlled, making it difficult to accurately perform a slip control. Furthermore, water hammering phenomenon, which occurs by pulsation of fluid when the plunger moves to open and close positions, results in a noise and deteriorates the durability of parts.
To solve the above described problems, a normal-open solenoid valve which is designed to reduce an amount of the fluid to be supplied to the wheel cylinder in proportion to a slip rate, while alleviating the pulsation of the fluid during a slip control, has been developed.
An example of such a normal-open solenoid valve is disclosed in U.S. Pat. No. 5,647,644.
As shown in FIG. 1, the normal-open solenoid valve disclosed in the patent comprises a valve seat 23, a magnetic core 21, a plunger 22, and a piston 24, all of which are installed in a valve housing 20. The valve seat 23 is forcedly fitted in a lower portion of the valve housing 20 and provided with a longitudinal hole. The plunger 22 is mounted through the magnetic core 21 such that a lower end thereof is disposed in the vicinity of a top of the valve seat 23. The piston 24 is disposed around the valve seat 23 and biased against the magnetic core 21 by a spring 25.
The normal-open solenoid valve is provided with two orifices, one of which is an invariable orifice 30 formed on an upper end of the valve seat 23 and the other is a variable orifice 40 that is defined by a slot 26 formed on a top of the piston 24 when the piston 24 contacts the magnetic core 21 during a slip control.
In addition, a hydraulic channel branch 31 is defined between one side wall of the valve seat 23 and the valve housing 20 so that hydraulic pressure supplied through an inlet 27 can be applied to the piston 24 through the hydraulic channel branch 31, thereby displacing the piston 24 upward while overcoming the biasing force of the spring 25. A stepped portion 23a is formed on another lower side of the valve seat 23 so as to forcedly fit the valve seat 23 into the valve housing 20.
In a normal brake mode, the plunger 22 is urged upward while the piston 24 is biased downward by the spring 25. Therefore, fluid is supplied through the inlet 27, then directed to an outlet 28 through the invariable orifice 30.
In a slip control mode, the plunger 22 is displaced downward to close the invariable orifice 30. As a result, the fluid supplied through the inlet 27 is applied to the piston 24 through the hydraulic channel branch 31 so that the top of the piston 24 contacts the magnetic core 21 by being moved upward while overcoming elastic force of the spring 25. After this, when the plunger 22 is moved upward, the hydraulic pressure generated by the master cylinder is directed to the wheel cylinder through the variable orifice 40 formed by the slot 26 of the piston 24 contacting the magnetic core 21.
When brake force is released, fluid within the wheel cylinder is returned to the master cylinder through the return passage 29 formed through the valve housing 20 to communicate the inlet 27 with the outlet 28. After this, the plunger 22 is moved upward so that the solenoid valve is returned to its open state.
However, in the above-described normal-open solenoid valve, since the lower portion of the valve seat is designed having the stepped portion and the channel branch defining portion, and the channel branch defining portion is precisely formed, it is difficult to manufacture the valve seat.
In addition, the special valve housing for receiving the magnetic core, the valve seat, the piston and the fluid passages is required, increasing the entire size of the solenoid valve.
Therefore, the present invention has been made in an effort to solve the above described problems.
It is an objective of the present invention to provide a solenoid valve for an anti-lock brake system, which can vary an amount of fluid to be supplied during an slip control.
It is another objective of the present invention to provide a solenoid valve, which is simple in the structure and easy to be manufactured.
To achieve the above objectives, the present invention provides a solenoid valve comprising a coil assembly disposed within a yoke to create an electromagnetic field using electric current applied, an armature disposed through a central axis of the yoke, the armature being slidable along the central axis by the electromagnetic field, a plunger coupled to the armature, a magnetic core provided with an invariable orifice opened and closed by the plunger to selectively communicate the fluid intake passage with the fluid exhaust passage, one end of the magnetic core being forcedly fitted in a bore of the modulator block, and a piston slidably disposed under the magnetic core to contact or move away from the invariable orifice formed in the magnetic core according to a brake mode, the piston being provided with a variable orifice which comes to communicate with the invariable orifice when the piston contacts the invariable orifice, a diameter of the variable orifice being smaller than that of the invariable orifice. The piston contacts the invariable orifice in a slip control mode, thus only fluid within the variable orifice is supplied to the wheel cylinder after passing through the invariable orifice and the fluid exhaust passage.
According to an embodiment of the present invention, the magnetic core is provided with a valve seat in which the invariable orifice is formed.
The piston is further provided with a piston passage coaxially extending from the variable orifice to communicate the fluid intake passage with the variable orifice and a plurality of branch passages branched off from the piston passage at a predetermined angle such that fluid supplied through the fluid intake passage can be directed to the invariable orifice in a normal brake mode.
Preferably, the predetermined angle is in a range of about 90-150 degrees, and at least one O-ring is provided around an outer circumference of the piston.
A spring is disposed between the magnetic core and the piston, the spring biasing the piston in a direction away from the invariable orifice of the magnetic core in a normal brake mode.
According to another embodiment of the present invention, the magnetic core is provided with a fluid outlet hole for directing fluid supplied through the invariable orifice to the fluid exhaust passage. A lip-seal is provided between an outer circumference of the magnetic core and the bore of the modulator block.
The solenoid valve may further comprise a fluid guide member fixedly fitted in the bore under the piston, the fluid guide member functioning as a means of guiding fluid supplied from the master cylinder to the piston as well as functioning as a stopper for restricting the downward movement of the piston.
A guide tube is formed in the fluid guide member to define a guide passage communicating with the fluid intake passage, a portion of the guide tube extending into the piston.
The guide tube is provided at a top with a first communicating hole for communicating the guide passage with the variable orifice, at a side with a plurality of second communicating holes for applying fluid to a lower end of the piston in a slip control mode.
A lip-seal is formed between an outer circumference of the fluid guide member and the bore of the modulator block.
According to still another embodiment of the present invention, the variable orifice is defined by a plurality of fluid grooves formed on a top of the piston, each width of the grooves is smaller than the diameter of the invariable orifice, and the piston is provided with a piston passage formed through a central axis and communicating the fluid intake passage with the variable orifice, and a plurality of branch passages branched off from the piston passage at a predetermined angle such that fluid supplied through the fluid intake passage can be directed to the invariable orifice in a normal brake mode.
The predetermined angle is in a range of about 90-150 degrees.
According to still yet another embodiment of the present invention, the magnetic core is provided with a bypass passage communicating with the fluid exhaust passage, the piston is provided with a return passage for communicating the bypass passage with the fluid intake passage, and a check valve is installed within the return passage.
Preferably, a filter is provided for holding back solid substances in fluid to be supplied.